1. Field of the Invention
This invention relates to a method of making a lithographic printing plate by the silver complex diffusion transfer process. In particular, this invention relates to a method of preventing developer mottle, toning and lack of transfer density when processing developer-incorporated plate material in fresh alkaline activator solutions.
2. Background of the Art
As described in various embodiments, e.g., U.S. Pat. Nos. 3,728,114, 4,160,670 and 4,361,635, diffusion-transfer imaging systems can provide a lithographic printing plate due to the oleophilic nature of the complexed silver in the receptor layer and the hydrophilic developed silver image in the emulsion layer. However, a very rapid development process is required to reduce exposed areas into nondiffusing silver images before the silver halide solvent can take effect. The greater the differentiation between the (unexposed) complexed silver and the (exposed) developed image area, the better the printing quality will be. However, the high pH required to produce such an active chemistry causes rapid oxidation of the developing agents. One method of overcoming this limitation is to incorporate the developing agents into the light sensitive medium and processing in an alkaline solution.
With respect to diffusion-transfer systems, in particular, a monosheet lithographic plate as described in U.S. Pat. No. 4,361,635 and EP 375,159, disclose a surface which consists of an extremely thin layer of physical developing nuclei such as palladium sol or salt thereof. If developing agents are incorporated into such a construction and the plate is machine processed in fresh alkaline activator, there is insufficient and uneven transfer development. The result is an oleophilic image that lacks printing endurance and has an undesirable bronze rather than shiny silver tone. Without wishing to be limited to any one particular theory, but it is assumed that these defects arise from the incorporated developer being eluted from the surface nucleating layer by the recirculating action of the processor before transfer development is complete.
According to G. M. Haist, Modern Photographic Processing, Vol. 2, John Wiley & Sons, 1979, p. 284-88, activators often require small quantities of developing agents in order to establish development equilibrium when machine processed. An example given by Haist is a lg/liter hydroquinone activator for obtaining optimum gamma in a fine-grain positive film.
U.S. Pat. No. 4,798,779 shows the use of aminophenols in silver salt diffusion transfer imaging systems. The aminophenol improves the black tone of the image.
G.B. 1,241,662 describes a developer incorporated diffusion-transfer plate. In some cases the patent specifically excludes any developing agents from the alkaline activator solution.
U.S. Pat. No. 4,298,673 describes making a lithographic printing plate using a diffusion-transfer developer containing a p-dihydroxybenzene and either a 3-pyrazolidone or an aminophenol.
U.S. Pat. No. 4,810,623 defines an "essentially developer-free" activator as one containing less than 0.014 mol./liter of developing agent. It specifically excludes diffusion-transfer systems.
E.P. 503,164 claims the use of a fresh alkaline activator solution containing between 10.sup.-4 to 5.times.10.sup.-2 mol./liter of p-dihydroxybenzene derivative(s) to make a printing plate by the silver halide diffusion-transfer process.
However, in the case of E.P. 503,164, the consistency of transferred image may be short lived due to the rapid oxidation of hydroquinone. While increasing the level of p-dihydroxybenzenes may overcome this limitation, it will result in a more intensely colored solution and increased precipitation of humic acids.